XI is a library. It is a set of ‘C’ functions and data structures that allows you to create user interfaces. XI is not a new language, resource compiler or editing tool. It is merely a function library and set of data structure definitions. Although written in C, XI works easily in C++ programs.

You use XI to create portable applications that display and edit data in a form or spreadsheet-style list. XI offers a variety of ready-made objects with predefined behaviors that can give an application a higher level of functionality and more sophisticated look and feel than you can have with XVT alone. Because XI was built using XVT, both the XI source code and your code written using the XI library will be portable across all platforms supported by XVT. Of course, you can also take full advantage of any XVT feature. Therefore, you use XI to expand the number tools you can use to create a portable application using XVT.

An XI application is composed of XI interfaces which contain XI objects. The objects are the spreadsheets, forms, edit fields, buttons, lines, and rectangles found in an XI interface. An XI interface is XI's object that contains information for a specific window. There is a one to one correspondence between XVT Windows and XI interfaces. To define an interface, you will be using certain XI functions. Other XI functions contain all of the functionality you need to write a XI application. You will encounter these functions when you are instantiating and manipulating the objects you’ve created. 

Portability 

XI sits on top of XVT as another layer. The purpose of XVT is to provide portability. XVT’s job is to unify the separate platforms under one programming interface. Because XI was written using the XVT functions and data structures, it achieves its portability as a direct result of using XVT. In addition, you may want to use the functionality provided by XVT. For example, you may want to have menus and modal dialog boxes or pie charts and graphs drawn with XVT drawing primitives. These are provided at the XVT level.